Data from: Experimental evidence does not support the habitat amount hypothesis

For a half century, habitat configuration – the arrangement of habitat patches within a landscape – has been central to theories of landscape ecology, population dynamics, and community assembly, in addition to conservation strategies. A recent hypothesis advanced by Fahrig (2013) would, if supported, greatly diminish the relevance of habitat configuration as a predictor of diversity. The Habitat Amount Hypothesis posits that the sample area effect overrides patch size and patch isolation effects of habitat fragmentation on species richness. It predicts that the amount of habitat in a local landscape, regardless of configuration, is the main landscape-level determinant of species richness. If habitat amount is indeed the major, landscape-level driver of species richness, the slopes of the species-area relationship (SAR) for otherwise similar fragmented and unfragmented landscapes should be indistinguishable. We tested the Habitat Amount Hypothesis with data from two replicated and controlled habitat fragmentation experiments that disentangle the effects of habitat amount and configuration. One experiment provided time-series data on plant species richness and the other on micro-arthropod species richness. We found that, relative to less fragmented habitats, the SARs for fragmented habitats have significantly higher slopes and that the magnitude of the difference in slopes increased over time. Relatively more species were lost in smaller areas when fragments were more isolated. In both experiments, the proportion of species lost due to increased habitat fragmentation was nearly identical to the proportion lost due to reduced habitat amount. Our results provide a direct and experimentally derived refutation of the Habitat Amount Hypothesis, supporting the long-held view that in addition to area, patch isolation and configuration are important determinants of species richness. Differences in species richness between fragmented and non-fragmented habitats increase over time, demonstrating that long-term studies are needed to understand the effects of fragmentation, above and beyond the amount of habitat lost.

半个世纪以来，生境配置（habitat configuration）——即景观中各生境斑块的排布方式——一直是景观生态学、种群动态、群落构建理论以及保护策略的核心研究议题。法里格（Fahrig）于2013年提出的一项新近假说若得到证实，将大幅削弱生境配置作为物种多样性预测因子的重要性。生境数量假说（Habitat Amount Hypothesis）认为，样方面积效应会掩盖生境破碎化对物种丰富度产生的斑块面积与斑块隔离效应。该假说预测，局部景观中的生境总量无论其配置如何，均是决定物种丰富度的核心景观层面驱动因素。倘若生境数量确实是调控物种丰富度的主要景观层面因子，那么在其他条件一致的破碎化与非破碎化景观中，种-面积关系（species-area relationship, SAR）的斜率应当难以区分。我们借助两项可重复且受控的生境破碎化实验数据对生境数量假说进行了检验，这两项实验能够有效区分生境数量与配置的独立效应。其中一项实验提供了植物物种丰富度的时间序列数据，另一项则针对微型节肢动物的物种丰富度展开了观测。研究结果显示，相较于破碎程度较低的生境，破碎化生境的种-面积关系斜率显著更高，且斜率差异的幅度随时间推移不断增大。当斑块隔离程度提升时，较小样地内的物种丢失量相对更多。在两项实验中，由生境破碎化加剧导致的物种丢失比例，与因生境总量减少引发的物种丢失比例几乎完全一致。我们的研究结果通过实验推导直接驳斥了生境数量假说，印证了长期以来的学术观点：除生境面积之外，斑块隔离与配置同样是决定物种丰富度的关键因素。破碎化生境与非破碎化生境之间的物种丰富度差异随时间持续扩大，这表明要理解生境破碎化的效应，除了考量生境丢失的总量之外，还需开展长期研究。